Allow passing single - argument as a means to accept file via STDIN

spec/ameba/cli/cmd_spec.cr

@@ -156,6 +156,11 @@ module Ameba::Cli
         c.globs.should eq %w[source1.cr source2.cr]
       end
 
+      it "accepts single '-' argument as STDIN" do
+        c = Cli.parse_args %w[-]
+        c.stdin_filename.should eq "-"
+      end
+
       it "accepts one unknown arg as explain location if it has correct format" do
         c = Cli.parse_args %w[source.cr:3:22]
 

src/ameba/cli/cmd.cr

@@ -78,7 +78,10 @@ module Ameba::Cli
       parser.on("-r", "--rules", "Show all available rules") { opts.rules = true }
       parser.on("-s", "--silent", "Disable output") { opts.formatter = :silent }
       parser.unknown_args do |arr|
-        if arr.size == 1 && arr.first.matches?(/.+:\d+:\d+/)
+        case
+        when arr.size == 1 && arr.first == "-"
+          opts.stdin_filename = arr.first
+        when arr.size == 1 && arr.first.matches?(/.+:\d+:\d+/)
           configure_explain_opts(arr.first, opts)
         else
           opts.globs = arr unless arr.empty?